.TH std::vector::operator[] 3 "2024.06.10" "http://cppreference.com" "C++ Standard Libary"
.SH NAME
std::vector::operator[] \- std::vector::operator[]

.SH Synopsis
   reference operator[]( size_type pos );             \fB(1)\fP (constexpr since C++20)
   const_reference operator[]( size_type pos ) const; \fB(2)\fP (constexpr since C++20)

   Returns a reference to the element at specified location pos. No bounds checking is
   performed.

.SH Parameters

   pos - position of the element to return

.SH Return value

   Reference to the requested element.

.SH Complexity

   Constant.

.SH Notes

   Unlike std::map::operator[], this operator never inserts a new element into the
   container. Accessing a nonexistent element through this operator is undefined
   behavior.

.SH Example

   The following code uses operator[] to read from and write to a std::vector<int>:


// Run this code

 #include <vector>
 #include <iostream>

 int main()
 {
     std::vector<int> numbers{2, 4, 6, 8};

     std::cout << "Second element: " << numbers[1] << '\\n';

     numbers[0] = 5;

     std::cout << "All numbers:";
     for (auto i : numbers)
         std::cout << ' ' << i;
     std::cout << '\\n';
 }

 // Since C++20 std::vector can be used in constexpr context:
 #if defined(__cpp_lib_constexpr_vector) and defined(__cpp_consteval)
 // Gets the sum of all primes in [0, N) using sieve of Eratosthenes
 consteval auto sum_of_all_primes_up_to(unsigned N)
 {
     if (N < 2)
         return 0ULL;

     std::vector<bool> is_prime(N, true);
     is_prime[0] = is_prime[1] = false;

     auto propagate_non_primality = [&](decltype(N) n)
     {
         for (decltype(N) m = n + n; m < is_prime.size(); m += n)
             is_prime[m] = false;
     };

     auto sum{0ULL};
     for (decltype(N) n{2}; n != N; ++n)
         if (is_prime[n])
         {
             sum += n;
             propagate_non_primality(n);
         }

     return sum;
 } //< vector's memory is released here

 static_assert(sum_of_all_primes_up_to(42) == 0xEE);
 static_assert(sum_of_all_primes_up_to(100) == 0x424);
 static_assert(sum_of_all_primes_up_to(1001) == 76127);
 #endif

.SH Output:

 Second element: 4
 All numbers: 5 4 6 8

.SH See also

   at access specified element with bounds checking
      \fI(public member function)\fP
